Origin of enhanced thermal atomic layer etching of amorphous HfO2

Abstract

HfO2 is a high- k material that is used in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of high- k materials at nanoscale device sizes. Thermal atomic layer etching (ALE) of metal oxides, in which up to one monolayer of material can be removed, can be achieved by sequential self-limiting fluorination and ligand-exchange reactions at elevated temperatures. First-principles-based atomic-level simulations using density functional theory can give deep insights into the precursor chemistry and the reactions that drive the etching of metal oxides. A previous study examined the hydrogen fluoride (HF) pulse in the first step in the thermal ALE process of crystalline HfO2 and ZrO2. This study examines the HF pulse on amorphous HfO2 using first-principles simulations. The Natarajan–Elliott analysis, a thermodynamic methodology, is used to compare reaction models representing the self-limiting and spontaneous etch processes taking place during an ALE pulse. For the HF pulse on amorphous HfO2, we found that thermodynamic barriers impeding spontaneous etching are present at ALE relevant temperatures. HF adsorption calculations on the amorphous oxide surface are studied to understand the mechanistic details of the HF pulse. An HF molecule adsorbs dissociatively by forming Hf–F and O–H bonds. HF coverages ranging from 1.1 ± 0.3 to 18.0 ± 0.3 HF/nm2 are investigated, and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. A theoretical etch rate of −0.82 ± 0.02 Å/cycle for amorphous HfO2 was calculated using a maximum coverage of 9.0 ± 0.3 Hf–F/nm2. This theoretical etch rate is greater than the theoretical etch rate for crystalline HfO2 that we previously calculated at −0.61 ± 0.02 Å/cycle. Undercoordinated atoms and void regions in amorphous HfO2 allow for more binding sites during fluorination, whereas crystalline HfO2 has a limited number of adsorption sites.

Science-based, data-driven developments in plasma processing for material synthesis and device-integration technologies

“…146) The thermal ALE process of crystalline HfO 2 and ZrO 2 using a hydrogen fluoride (HF) pulse was calculated using a computational model. 147) 2.8.2. Minimization of plasma-induced damages.…”

Fluorination of TiN, TiO2, and SiO2 Surfaces by HF toward Selective Atomic Layer Etching (ALE)

“…Using atomistic computational chemistry simulations, the pathways of each reaction in the etching processes can be explored, and volatile byproducts can be inferred [23,24]. However, since the thermal ALE processes present various theoretical challenges, such as self-limiting reactions and the generation of volatile reaction products, only limited studies have been conducted to identify the reaction mechanisms [25][26][27][28][29][30][31][32]. In particular, while the process conditions for fluorination and removal of Ti atoms from TiO 2 by HF were previously molecularly elucidated [33,34], its selectivity against TiN or SiO 2 substrates is yet to be known.…”

Selective etching mechanism of silicon oxide against silicon by hydrogen fluoride: a density functional theory study

“…DFT calculations using slab models were also used to study the fluorination of metal oxides by HF molecules, such as Al 2 O 3 , 19 TiO 2 , 20 ZrO 2 , 21 and HfO 2 . 21,22 To date, the complete simulation of the etching reactions of SiO 2 and Si by HF using surface slab models has not been reported, and the comparison between SiO 2 and Si to understand the origin of selectivity is lacking.…”